Field of the Invention
The present invention relates to improved arrangements for providing thermal connection between a cryogenic refrigerator and cooled components, wherein the refrigerator is removable, and the thermal connection must be capable of being broken and re-made without discernible increase in thermal resistance.
The present invention is particularly described in the context of a two-stage cryogenic refrigerator cooling to temperatures of about 4.2K for re-condensing helium in a cryostat used for cooling superconducting magnets for MRI systems.
Description of the Prior Art
FIG. 1 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage 30 is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage 32 provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K. In current cryogenic refrigerators, the first stage may provide about 44 W of cooling to 50K and about 1 W of cooling at about 4K.
A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20.
U.S. Pat. Nos. 4,667,487, 4,986,077, JP H05 245394A describe conventional arrangements for mounting a cryogenic refrigerator.
The present invention is particularly concerned with mounting arrangements for cryogenic refrigerator 17 and its interface with refrigerator sock 15.
A first stage 30 of the refrigerator 17 is generally pressed into contact with a first stage of the sock. That first stage of the sock is generally in thermal contact with thermal radiation shield 16. At a lower, closed, end of the sidesock, a second stage 32 of the refrigerator is provided. When in position, the second stage 22 of the refrigerator 17 may be pressed into contact with a second stage of the sock 15. The second stage of the sock is typically thermally linked to a heat exchanger which is exposed to gaseous cryogen in the cryogen vessel 12. In some arrangements, the heat exchanger is exposed directly to the interior of the cryogen vessel. In other arrangements, the heat exchanger is positioned within a small recondensing chamber, which is linked to the main cryogen vessel by one or more passageways.
In such arrangements, it is important to have a suitable mechanical pressure on both first and second stages of the refrigerator, to provide effective thermal contact between stages of refrigerator 17 and stages of sock 15 which must be maintained when in use at cryogenic temperatures.
Refrigerator sock 15 may have a flexible connection of some sort built in, in an attempt to ensure effective mechanical connection despite variations in component sizes due to build tolerances.
The first and second stages of the refrigerator 17 are more clearly visible in FIG. 2. In case of insufficient thermal contact between refrigerator and sock, effective cooling will not be provided to the thermal radiation shield and the heat exchanger; and it may not be possible to maintain the required temperature within the cryogen vessel. For example, a hard mechanical contact may be employed, in which the second stage heat exchanger 32 is pressed into mechanical contact with a heat exchanger. This is typically arranged by careful selection of the length of the sock 15 particularly the distance between first and second stages of the sock to correspond to the distance between first and second stages of the refrigerator. Thermal contact between the first stage of the refrigerator and the first stage of the sock may be achieved by direct mechanical contact, in which the first stage of the refrigerator and the first stage of the sock are provided by solid metal pieces with complementary tapers. Due to dimensional variation inherent in the manufacturing processes, it is difficult to reliably achieve an appropriate mechanical pressure between the second stage of the refrigerator and a second stage of the sock, arranged in contact with the thermal bus bar as well as an appropriate mechanical pressure between the first stage of the refrigerator and the first stage of the sock. If mating faces of the stages of the refrigerator and the stages of the sock are not accurately formed due to assembly tolerances, then the thermal contact surface area, and therefore recondensing performance, may be reduced. The second stage of the sock is typically placed at the closed end of the sock, and so the distance between the first stage of the sock and the second stage of the sock is fixed during construction of the sock. It must also be possible to remove the refrigerator from the sock for servicing and replace or substitute it, yet achieve an acceptable thermal contact with the thermal bus bar when the refrigerator is re-installed.
FIG. 13 shows an example prior art arrangement, as described in US2005/0166600, where a cryogenic refrigerator R having a first stage H1 and a second stage H2 is located within a sock 2 itself having a first stage F1 and a second stage F2. In order to make effective thermal joints between respective first and second stages, pressure is applied to an upper flange 4 of the refrigerator, typically by bolting the upper flange to a mounting point F3 at the top of the sock, attached to the cryostat 100. This presses the refrigerator into the sock, and provides contact pressure between the first stage H1 of the refrigerator and the first stage F1 of the sock; and between the second stage H2 of the refrigerator and the second stage F2 of the sock. Depending on build tolerances of the various components concerned, the distribution of contact force between first and second stages will vary. It may be found prudent to provide an indium washer 3a, 3b or a layer of thermally conductive grease between the refrigerator and the sock at each stage, but such indium washers or grease are difficult to remove when a refrigerator is removed for servicing and replaced. More significantly, a relatively large force is applied to the flange 4, which places a compressive force on the refrigerator, and a tensile force of the sock. The refrigerator R is a fragile precision machine, and it would be preferable to avoid placing significant forces on the body of the refrigerator.